When parties to a contract make promises to perform their obligations, and one party reasonably relies on the other party’s promise—but the party making the promise fails to perform, causing harm or loss to the party who relied on the promise—the party who relied on the promise to perform is said to have relied to its detriment.

This legal concept is called detrimental reliance. Detrimental reliance may serve as a substitute for consideration, and make an otherwise unenforceable contract enforceable.

Thus, detrimental reliance is a legal concept based on fairness (known as equity or equitable), and is equivalent to contractual promissory estoppel (due to the other party’s reliance, the party who did not keep its promise is prohibited from challenging the enforceability of its promise).

Detrimental reliance is not a separate tort cause of action.

In Washington State, the legal concept of detrimental reliance, also known as promissory estoppel, is recognized and can be used to enforce a contract that may otherwise lack consideration and be unenforceable. This doctrine applies when one party makes a promise that the other party relies upon to their detriment. If the relying party's reliance was reasonable and it resulted in harm or loss due to the promisor's failure to perform, the court may enforce the promise to prevent injustice. The principle is grounded in equity, aiming to uphold fairness in contractual relationships. While detrimental reliance is not a separate tort cause of action, it is a legal remedy that can be invoked in civil disputes related to contractual matters. An attorney can provide specific guidance on how this doctrine might apply in a particular case in Washington.